THEREDA – The Thermodynamic Reference Database for a Nuclear Waste Disposal in Germany

The disposal of nuclear waste including the assessment of long-term safety is still an open question in Germany. In addition to the still pending decision about the repository host rock (salt, granite, or clay) the basic necessity of a consistent and obligatory thermodynamic reference database persists. Such a database is essential to assess potential failure scenarios accurately and to make well founded predictions about the long-term safety. Specific needs for waste repository and remediation projects in Germany are comprehensive datasets also covering high temperatures and high salinities. Against this background, available databases do not suffice and are limited in their use, partly because of high restrictions and resulting incompleteness of reactions. Other databases rely on heterogeneous and therefore inconsistent data leading to incorrect model calculations. Due to these deficiencies THEREDA, a joint project of institutions leading in the field of safety research for nuclear waste disposal in Germany, was started in 2006 [1,2].
THEREDA contains a relational databank whose structure has been designed in a way that promotes the internal consistency of thermodynamic data. Data considered cover the needs of Gibbs Energy Minimizers and Law-of-Mass-Action programs alike. Parameters for a variety of models describing interactions in mixed phases are included. Namely the Pitzer- and SIT-parameters to describe activity coefficients of hydrated ions and molecules are considered. Provision is made to include parameters for solid solution models and non-ideal gas models at a later stage. Both thermodynamic and interaction parameters can be described by temperature functions.
THEREDA offers evaluated thermodynamic data for all compounds (solid phases, aqueous species, or constituents of the gaseous phase) of elements, which according to the present state of research are relevant. In particular, all oxidation states expected for disposal site conditions are covered.
The guidelines developed by the NEA form the basis for those for THEREDA. They were extended to cover also areas such as nomenclature, interaction coefficients, temperature and pressure functions, and quality assurance (QA). The latter contains an internal review where correctness, plausibility an completeness of documentation are checked.
An important step is the data assessment. It involves subdividing the data according to

• Class: distinction between “real” datum, chemical analogue, or estimated value
• Category: type of experiment from which the datum was derived
• Quality: reliability of the datum derived from its numerical uncertainty
• Source: type of literature from which the value was adopted (international reviews, based on the analysis of many papers, internal value…)

Parameter files are created from the databank in a variety of formats (generic ASCII type, and formats required by the geochemical speciation codes PhreeqC, EQ3/6, ChemApp and Geochemist’s Workbench). They are also used for internal benchmark calculations – another essential element of the QA scheme. The results are documented and provided to the user.
THEREDA is accessible via internet through www.thereda.de. This is not only a portal to the database, but shall also serve as an information and discussion platform on issues concerning the database. Thus we are confident to generate helpful feedback from the anticipated user community.
[1] Altmaier, M.; Brendler, V.; Hagemann, S.; Herbert, H.-J.; Marquardt, C.; Moog, H.; Neck, V.; Richter, A.; Voigt, W.; Wilhelm, S. (2008). „THEREDA - Ein Beitrag zur Langzeitsicherheit von Endlagern für nukleare und nichtnukleare Abfälle.“ ATW 53, 249-253.
[2] W. Voigt, V. Brendler, K. Marsh, R. Rarey, H. Wanner, M. Gaune-Escard, P. Cloke, Th. Vercouter, E. Bastrakov, S. Hagemann (2007), „Quality assurance in thermodynamic databases for performance assessment studies in waste disposal”, Pure Appl. Chem. 79, 883-894.